This invention relates to a new class of materials generally comprising a core dendrimer molecule and a plurality of shell dendrimer molecules chemically bonded to the surface of the core dendrimer molecule.
Dendritic macromolecules have received substantial attention during the past two decades on account of their unusual structural and chemical properties, such as their almost perfect isomolecularity (i.e., polydispersity near unity, e.g., 1.0003 to 1.0005 for dendrimers having a molecular weight of about 10,000), their well defined morphology (i.e., uniform size and shape), and their unusually high chemical functionality; and because of their actual and potential utility in diverse applications, such as in biomedicine, pharmaceuticals, personal care, industry, material science, materials engineering, and research. Known dendritic structures include various dendrimers, regular dendrons, controlled hyperbranched polymers, dendrigrafts, random hyperbranched polymers, bridged dendrimers, and others.
It is widely recognized that dendrons and dendrimers constitute a significant subclass of xe2x80x9cdendritic polymersxe2x80x9d and represent a unique combination of very high structural complexity, together with extraordinary structural control. The assembly of reactive monomers, branch cells or dendrons around atomic or molecular cores to produce dendrimers according to divergent or convergent dendritic branching principles is well known (see for example M. K. Lothian-Tomalia, D. M. Hedstaand, D. A. Tomalia, A. B. Padias, and H. K. Hall, Jr., Tetrahedron 53, 15495 (1997); D. A. Tomalia, A. M. Naylor and W. A. Goddard III, Angew. Chem. Int. Ed. Engl. 29(2), 138 (1990); and C. J. Hawker and J. M. J. Frechet, J. Am. Chem. Soc. 112, 7638 (1992). Systematic filling of space around cores with branch cells, as a function of generational growth stages, to provide discrete, quantized bundles of mass has been shown to be mathematically predictable (see D. A. Tomalia, Adv. Mater. 6, 529 (1994); P. R. Dvornic, D. A. Tomalia, Chemistry In Britain, 30(8), 641 (1994) and P. R. Dvornic, D. A. Tomalia, Macromol. Symp., 98, 403 (1995)). Predicted theoretical molecular weights have been confirmed by mass spectroscopy and other analytical methods. The resulting dendritic architectures have allowed the systematic control of molecular structural parameters such as size, shape, surface functionality, and interior functionality at the lower end of the nanoscale region, e.g., from about 1 to about 15 nanometers. It is well established that dendritic structures can be utilized to define confined spaces at the lower end of the nanoscale region (see for example R. Esfand and D. A. Tomalia, Chemistry and Industry, 11, 416 (1997)).
Divergent dendritic growth can be precisely controlled to form ideal dendritic polymers which obey mathematical formulas, at least through the first several generations of growth. However, because the radii of dendrimer molecules increase in a linear manner as a function of generation during ideal divergent growth, whereas the surface cells amplify according to geometric progression law, ideal dendritic growth does not extend indefinitely. There is a critical generation at which the reacting dendrimer surface does not have enough space to accommodate incorporation of all of the mathematically required new units. This stage in ideal dendritic growth is referred to as the de Gennes dense-packed stage. At this stage, the surface becomes so crowded with terminal functional groups that, although the terminal groups are chemically reactive, they are sterically prohibited from participating further in ideal dendritic growth. In other words, the de Gennes dense-packed stage is reached in divergent synthesis when the average free volume available to the reactive surface group decreases below the molecular volume required for the transition state of the desired reaction to extend the growth to the next generation. Nevertheless, the appearance of the de Gennes dense-packed stage in divergent synthesis does not preclude further dendritic growth beyond this point. It has been demonstrated by mass spectrographic studies that further increase in the molecular weight can occur beyond the de Gennes dense-packed stage. In the case of an ammonia core polyamidoamine (PAMAM) dendrimer, dendritic growth has been observed to generation 12. However, at that generational level, experimentally observed molecular weights are lower than the mathematically calculated values. Products resulting from continuation of dendritic growth beyond the dense-packed stage are xe2x80x9cimperfectxe2x80x9d in structure, because some of the surface groups in the precursor generation are sterically precluded from undergoing further reaction. The number of functional groups on a dendrimer which has been grown past the de Gennes dense-packed stage will not correspond to the ideal, mathematically predicted value for that generation. In the case of PAMAM dendrimers (NH3 core), a gradual digression from theoretical masses occurs for generations 5-8, followed by a substantial break (i.e., about 23%) between generation 8 and generation 9 PAMAM dendrimers. This discontinuity is interpreted as a signature for the de Gennes dense-packed stage. It should be noted that the digression from theoretical masses for successive generations continues to decline monotonically with each successive generation beyond generation 9. A similar trend is noted for other dendrimers.
In addition to the deviation from expected molecular weight, the interior of dendritic polymers of higher generations beyond the de Gennes dense-packed stage becomes less accessible to guest molecules. For example, it has been shown that the interior of hydroxyl terminated (ethylene diamine core) PAMAM dendrimers from generation 1 to generation 6 are completely accessible to copper(II) ions, whereas the interior of generation 7 through 10 PAMAM dendrimers is not accessible to copper(II) ions. Subsequent treatment of solutions containing copper(II) ions and generations 1 through 10 dendrimers, with hydrogen sulfide resulted in copious precipitates for solutions containing PAMAM dendrimers of generations 1 through 3, completely soluble solutions with generations 4 through 6, and precipitates with generations 7 through 10. Analysis (TEM) of the solutions containing generations 4 through 10 PAMAM dendrimers confirmed that the generations 4 through 6 dendrimers contained copper sulfide within their interiors, and were functioning as host container molecules. Similar analyses indicated that generations 7 through 10 dendrimers were functioning as surface scaffolding for copper(II) ions, with essentially no copper in the interior.
In various applications, particularly in biomedical applications, it may be desirable to provide larger dendritic polymer structures, having a high loading capacity (i.e., the ability to contain drugs and/or diagnostic compounds), and having relatively predictable sizes, shapes and chemical valency to ensure consistent, controllable performance. However, the largest known dendrimer diameter is about 30 nanometers. The larger dendrimers have relatively dense surfaces which can be impermeable to a variety of drugs and/or diagnostic compounds. Accordingly, the ability of the higher generation dendrimers to act as carriers for drugs and/or diagnostic compounds is generally limited to their ability to retain such compounds by coordination with surface functional groups.
Although bridged dendrimers and dendrimer clusters are known, these structures are generally limited to random branched and/or crosslinked structures of non-uniform size, shape and polydispersity. Accordingly, known bridged dendrimers, crosslinked dendrimers, and dendrimer clusters do not exhibit sufficient predictability, regularity and/or uniformity for use in certain applications, such as a carrier for drug delivery and/or delivery of diagnostic compounds.
U.S. Pat. No. 4,737,550 discloses bridged dense star polymers. The patent discloses that the bridged dense star polymers are comprised of at least two dense star polymer molecules which are covalently bonded together, preferably through terminal groups of the dense star polymer molecules. The patent does not provide any specific disclosure of a core-shelled dendritic polymer molecule of the type disclosed herein, and does not teach how core-shell dendritic polymer molecules of the type disclosed herein can be made. It is generally disclosed in the patent that excess dendrimers with nucleophilic terminal groups may coat a dense star or dendrimer seed.
Therefore, there remains a need for larger dendritic structures having relatively precisely defined, uniform structure, shape and size.
The present invention provides a composition of matter comprising the reaction product of a core dendritic polymer molecule having a plurality of terminal functional groups of a first type which are not reactive with each other and a plurality of shell dendritic polymer molecules having a plurality of terminal functional groups of a second type which are not reactive with each other, but which are reactive with the terminal functional group of the first type, and processes for preparing the composition of matter. The shell dendritic polymer molecules are chemically bonded to the core dendritic polymer molecule by a reaction of at least one of the terminal functional groups of the second type with at least one of the terminal functional groups of the first type. The reaction product comprises at least 40% of the theoretical maximum number (Mm) of shell dendritic polymer molecules that can be bonded to the core dendritic polymer molecule, wherein             M      m        =                            2          ⁢                      xe2x80x83                    ⁢          π                          3                    ⁢                        (                                                    r                1                                            r                2                                      +            1                    )                2              ,
where r1 is the radius of the core dendritic polymer molecule and r2 is the radius of the shell dendritic polymer molecule.
In a particular aspect of the invention, the composition of matter is the reaction product of a single dendritic core polymer molecule, a plurality of shell dendritic polymer molecules chemically bonded to the surface of the core dendritic polymer molecule to form a first shell surrounding the core molecule, and a plurality of dendritic polymer molecules bonded to the shell dendritic polymer molecules to form a second shell surrounding the first shell.
The process for preparing the composition of matter comprising the reaction product of a core dendritic polymer molecule and a plurality of shell dendritic polymer molecules surrounding and chemically bonded to the core dendritic polymer molecule comprises providing the core dendritic polymer molecules, providing the shell dendritic polymer molecules, and combining core dendritic polymer molecules with the shell dendritic polymer molecules in a solution. The dendritic core polymer molecules each have a plurality of terminal functional groups of a first type which are not reactive with each other, and the shell dendritic polymer molecules have a plurality of terminal functional groups of a second type which are not reactive with each other, but which are reactive with the terminal functional groups of the first type. The amounts of core dendritic polymer molecules and the shell dendritic polymer molecules are selected so that the ratio of shell dendritic polymer molecules to core dendritic polymer molecules is at least 4:1. After the core dendritic polymer molecules and the shell dendritic polymer molecules are combined, the terminal groups of the first type are reacted with the terminal groups of the second type to form a core-shell polymer which is the reaction product of a single core dendritic polymer molecule and a plurality of shell dendritic polymer molecules that are chemically bonded to, and surround, the core dendtritic polymer molecule. The number of shell dendritic polymer molecules bonded to each core dendritic polymer molecule is at least 40% of the theoretical maximum number (Mm) that can be bonded to the core dendritic polymer molecule, wherein             M      m        =                            2          ⁢                      xe2x80x83                    ⁢          π                          3                    ⁢                        (                                                    r                1                                            r                2                                      +            1                    )                2              ,
where r1 is the radius of the core dendritic polymer molecule and r2 is the radius of the shell dendritic polymer molecule.
The preferred core-shell dendritic polymers of this invention are believed to represent a new class of materials comprising a single core dendritic polymer molecule surrounded by a plurality of shell dendritic polymer molecules, each of which is chemically bonded to the surface of the core dendritic polymer molecule, to form a shell or outer layer around the core dendrimer molecule. The resulting core-shell structure is thus formed from a plurality of elemental units (i.e., the core dendritic polymer molecule and the plurality of shell dendritic polymer molecules bonded to the core dendritic polymer molecule) assembled in a relatively predictable manner. The resulting reaction product has been termed a core-shell tecto(dendrimer) by the inventors.
The core-shelled dendritic polymers of this invention are characterized by, and differ from, known dendritic polymer structures, including bridged dense star polymers, by their high degree of uniformity in size, shape and polydispersity. Unlike the random clusters of the prior art, the core-shelled dendritic polymers of this invention are characterized by a single core dendrimer surrounded by and chemically bonded to at least 40% of the theoretical maximum number of shell dendritic polymer molecules that can be bonded to the core, the theoretical maximum number (Mm) of shell dendritic polymer molecules that can be bonded to the core dendritic polymer molecule being represented by the expression             M      m        =                            2          ⁢                      xe2x80x83                    ⁢          π                          3                    ⁢                        (                                                    r                1                                            r                2                                      +            1                    )                2              ,
where r1 is the radius of the core dendritic polymer molecule and r2 is the radius of the shell dendritic polymer molecule.
The core dendritic polymer molecule and the shell dendritic polymer molecules can be selected from a variety of dendritic polymers, provided that the core dendritic polymer molecule includes terminal functional groups that are reactive with terminal functional groups of the shell dendritic polymer molecule.
Although it is conceivable that the core-shell tecto(dendrimers) can be prepared from random hyperbranched polymers, controlled hyperbranched polymers (dendrigrafts), and other non-ideal dendritic polymer molecules, ideal dendrimers and nearly ideal dendrimers, especially those having a nearly spherical or slightly elliptical shape are preferred. The dendritic polymer macromolecules useful in the practice of this invention are characterized by a relatively high degree of branching, which is defined as the number average fraction of branching groups per molecule (i.e., the ratio of terminal groups plus branch groups to the total number of terminal groups, branch groups and linear groups). For ideal dendrons and dendrimers, the degree of branching is 1. For linear polymers, the degree of branching is 0. A degree of branching of at least about 0.5 is desirable, and a degree of branching of at least about 0.9 is preferred.
The invention encompasses core-shell tecto(dendrimers) which are the reaction products of higher generation, higher molecular weight dendrimers beyond the de Gennes dense-packed stage, i.e., dendritic polymers in which surface groups are sterically prohibited from participating in further dendritic growth and in which the surface of the dendritic polymer is characterized by low permeability. However, lower generation, lower molecular weight dendrimers exhibiting good container properties and good surface permeability are preferred. In the case of PAMAM dendrimers having an ammonia core, preferred generations include generations 3 through 8. Lower generation (2.5 or less) PAMAM dendrimers having an ammonia core are not as highly preferred because of their more elongate shape, and because of their relatively poor container properties. Higher generation (8.5 or more) PAMAM dendrimers with an ammonia core are useable, but are not as highly preferred because of their relatively low surface permeability and poor container properties. Preferred ethylenediamine core PAMAM dendrimers include generations 3 through 7, although higher generation or lower generation ethylene diamine core PAMAM dendrimers can be used.
A preferred method for preparing the core-shell tecto(dendrimers) is by preparing, or obtaining, a first solution containing dendritic polymer molecules having a plurality of terminal functional groups of a first type which are not reactive with each other; providing a second solution containing dendritic polymer molecules having a plurality of terminal functional groups of a second type which are not reactive with each other, but which are reactive with the terminal functional groups of the first type; and combining the first and second solutions under conditions in which the terminal groups of the first type react with the terminal groups of the second type to form a core-shell reaction product. The reaction products can be prepared under a variety of conditions, but are preferably prepared at moderate temperatures, i.e. from about room temperature to about 50xc2x0 C., and at a moderate pressure, e.g., at about normal atmospheric pressure. A suitable reaction time is from several hours to several days.
The core-shell tecto(dendrimers) of this invention may be prepared by reacting a core dendrimer having functional groups of a first type which are not chemically reactive with each other, with an excess of shell dendrimers having functional groups of a second type which are not chemically reactive with each other or with the functional groups of the first type, but which interact or associate with the functional groups of the first type to form a self-assembled complex. After a desired self-assembled complex has been formed, the shell dendrimers can be bonded to the core dendrimer by addition of a suitable reagent (e.g., carbodiimide). The reaction is preferably carried out in a relatively dilute solution to prevent agglomeration and formation of clusters having non-uniform properties.
In the case of the reaction of a core dendrimer having amine terminal groups, with shell dendrimers having methyl ester terminal groups, the reaction may be carried out in the presence of lithium chloride, which tends to act as a catalyst, i.e., as an agent which accelerates the reaction.
A preferred method of preparing core-shell dendrimers involves the reaction of a core dendrimer having carboxylic acid terminal groups, with shell dendrimers having amine terminal groups, in the presence of lithium chloride, which is present in an equal molar quantity based on the number of terminal groups of both types. In the presence of lithium chloride, it is believed that the terminal groups become ionically bonded to one another, rather than covalently bonded. It is believed that this allows the terminal groups to become loosely associated with each other, thus allowing the shell dendrimers to become displaced by other shell dendriners contacting the surface of the core dendrimer. This process allows realignment or adjustment of the shell dendrimers on the surface of the core dendrimer to give a more ideal self-assembled complex. This results in more efficient packing or filling of the shell dendrimers around the core dendrimer. After a sufficient period of time, e.g., 12-18 hours, carbodiimide is introduced. The carbodiinide causes the amine groups of the shell dendrimers to become covalently bonded to the carboxylic acid terminals on the core dendrimer. The result is more ideal (greater) filling of the shell, i.e., the number of shell dendrimers chemically bonded to the core dendrimer approaches the theoretical maximum.
The theoretical maximum number of spheres of a particular size which can be bonded or attached to a core sphere of a particular size, and the average number of spheres of a particular size which can randomly attach to a core sphere of a particular size are discussed in M. L. Mansfield, L. Rakesh and D. A. Tomalia, The Random Parking Of Spheres On Spheres, J. Chem. Phys., 105(8), Aug. 22, 1996. When the core dendritic polymer molecule and the shell dendritic polymer molecules are treated as perfect spheres, the theoretical maximum number of shell spheres which can be attached to the core sphere (Mm) is given by the mathematical expression             M      m        =                            2          ⁢                      xe2x80x83                    ⁢          π                          3                    ⁢                        (                                                    r                1                                            r                2                                      +            1                    )                2              ,
where r1 is the radius of the core sphere, and r2 is the radius of each of the shell spheres. The radii of many dendritic polymers have been determined and published. For example, see Catheryn L. Jackson et al., Visualization Of Dendrimer Molecules by Transmission Electron Microscopy (TEM): Staining Methods and Cryo-TEM of Vitrified Solutions, Macromolecules, 31(18), 6259. However, in those cases where literature values are unavailable or suspect, radii can be accurately determined using a variety of analytical techniques including SEC, TEM, TPC, light-scatting, atomic force microscopy, etc.
As previously stated, it is desirable that the reaction to form the core-shell tecto(dendrimers) of this invention be carried out with an excess of shell dendrimers. Desirably, the reaction is carried out under conditions wherein the ratio of shell dendrimers to core dendrimers is at least 4, and more preferably at least 10. Desirably, the number of shell dendrimers chemically bonded to the core dendrimer is at least 25%, more desirably at least 40% or 50%, and even more desirably at least 75% of the theoretical maximum, as determined mathematically based on treating the dendrimers as perfect spheres.
In addition to forming core-shell tecto(dendrimers) comprising a single shell or layer of dendrimers of a second type chemically bonded to a core dendrimer, the invention also encompasses multiple layers or multiple shell tecto(dendrimers). For example, a core-shell dendrimer can be refunctionalized to form a core-shell tecto(dendrimer) having terminal functional groups of a first type, and subsequently reacted with dendrimers having functional groups of a second type, in which the functional groups are not reactive with functional groups of the same type, but in which the functional groups of the second type are reactive with the functional groups of the first type. In this manner, a second shell can be formed over the core-shell tecto(dendrimer). This process can be repeated a plurality of times to form generally any desired number of shells. As a specific example, a core comprising a dendrimer with amine terminal groups can be reacted with a plurality of shell dendrimers having methyl ester groups to form a core-shell tecto(dendrimer) having methyl ester terminal groups. Subsequently, the core-shell tecto(dendrimer) can be reacted with an excess of a polyamine, such as ethylene diamine, to form a core-shell tecto(dendrimer) having amine terminal groups. The amine terminated core-shell tecto(dendrimer) can be subsequently reacted with an excess of methyl ester terminated dendrimer molecules, with the ester terminated dendrimer molecules forming a second shell over the first shell. The process can be repeated to form additional shells as desired. As another specific example, a carboxylic acid terminated dendrimer can be prepared by reacting a methyl ester terminated dendrimer with an excess of succinic anhydride. An excess of the resulting carboxylic acid terminated PAMAM dendrimers can be reacted with an amine terminated core dendrimer to form a carboxylic acid terminated core-shell tecto(dendrimer). The carboxylic acid terminated core-shell tecto(dendrimer) can be reacted with an excess of a diamine, such as ethylene diamine to form an amine terminated core-shell tecto(dendrimer) which can be subsequently reacted with an excess of another carboxylic acid terminated dendrimer to form a second layer or shell over the first shell. Again, the procedure can be repeated as desired to form any number of layers.